Along with the higher performance and more sophisticated functions of automobiles, the performance required to tires has become greater with each passing year. Development in tires, while maintaining the grip on a wet road surface, that is, the wet grip, providing low fuel consumption has been strongly desired, as one of the requirements. The reinforcing filler which had been used for tire treads in the past was carbon black, but recently, due to the above demand, ultrafine silcia particles having a better low hysterisis loss and wet skid resistance performance, compared with carbon black, have begun to be used as a reinforcing filler for tire treads (e.g., see U.S. Pat. No. 5,227,425). However, silica-based fillers have hydrophilic silanol groups on the surface thereof, and therefore, is inferior in affinity with rubber molecules, compared with carbon black. Thus, silica-based fillers are superior in the low hysteresis and wet skid performance, but have the problem that the reinforcibility or abrasion resistance are not sufficient when compared with carbon blacks. Therefore, to make the reinforcibility of silica-based fillers to the same extent as that of carbon black, a silane coupling agent chemically bonding to rubber molecules and the surface of silica particles and capable of increasing the reinforcibility has been jointly used (e.g., see U.S. patent application Ser. No. 467,583). As a representative silane coupling agent, bis(3-triethoxysilylpropyl) tetrasulfide may be mentioned.
However, there is the problem that, when a silane coupling agent is compounded into a silica-containing rubber composition, if the coupling reaction between the silica and the silane coupling agent is insufficient, a good dispersibility of silica cannot be obtained, while if the coupling reaction is excessive, rubber scorching is caused to decrease the quality of the rubber composition. Therefore, according to the conventional kneading method, the rubber temperature is constantly measured by a thermocouple, etc. during the kneading operation, the rubber temperature is maintained in a constant range and the kneading operation is carried out within an empirically set period of time, but the amount of reaction of the silica and silane coupling agent does not necessarily have to be constant in each batch of the product. Thus, balancing the mixability and proccessability of silica with respect to the desired rubber properties is very difficult.
As a matter of fact, in silica-containing rubber compositions, diphenyl guanidine (DPG) is frequently used, as a conventional vulcanization accelerator. In recent years, its toxicity and the adverse effects on rubber steel cord bonding have become a concern. There is, therefore, a movement to decrease the using amount of DPG. However, in a rubber composition containing a large amount of silica, there was the problem that, if the using amount of DPG is decreased, the vulcanization rate is decreased and the dispersion of the silica in the rubber is deteriorated.
Japanese Patent Publication (A) No. 2005-112921 discloses that by compounding a secondary amine compound having a piperidine skeleton, i.e., 2,2,6,6-tetramethylpiperidine or its derivative into a diene-based rubber, it is possible to obtain a rubber composition having a high grip performance, but this publication does not describe the compounding of this compound together with silica, and therefore, there are no description as to the dispersability or processability of silica.
Japanese Patent Publication (A) No. 2006-509851 describes a vulcanized product exhibiting excellent mechanical properties, while maintaining the allowable vulcanization rate even when no secondary vulcanization accelerator is added, is obtained by compounding an organic quaternary ammonium salt into a diene elastomer polymer, together with silica or the other additives to a rubber composition. This Publication describes the use of 1,4-diazabicyclo[2,2,2]octane for preparation of an organic quaternary ammonium salt (see Examples 1 and 2), but does not describe at all that 1,4-diazabicyclo[2,2,2]octane is compounded into a rubber composition.